Fix assignments of null strings in QML and JavaScript

Assigning "" to a string based property would exhibit inconsistent behavior:

  * When assigned as a literal, it would assign an empty string
  * When assigned from JavaScript, it would assign a null string

When the assignment was done _to_ a QByteArray property, it would hit
the case of calling QVariant::convert where the incoming variant is
either an empty or a null string and the target is a QByteArray. For historical
reasons - as documented - QVariant::convert will return false when the
incoming variant is a null variant.

In V8 assignment from JavaScript would produce an empty string and thus
hit the "succesfull" conversion code path in QVariant to convert to a
seemingly empty QByteArray. With v4 a null string would result in a failed
conversion and spurious warnings as seen in the reported task.

This patch ensures that we consistently map "" to a null string, when it comes
from JavaScript or QML as a literal string. We now also detect the situation of
trying to convert a (valid) null variant to another target type.

Task-number: QTBUG-37197
Change-Id: I68f9031262fdd287d69a38d5468fb38a20441d7b
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

2 files changed, 8 insertions, 0 deletions

diff --git a/src/qml/compiler/qv4compileddata_p.h b/src/qml/compiler/qv4compileddata_p.h
index afcf2c6a24..a4dcfd9209 100644
--- a/src/qml/compiler/qv4compileddata_p.h
+++ b/src/qml/compiler/qv4compileddata_p.h
@@ -196,6 +196,8 @@ struct Unit
         const uint *offsetTable = reinterpret_cast<const uint*>((reinterpret_cast<const char *>(this)) + offsetToStringTable);
         const uint offset = offsetTable[idx];
         const String *str = reinterpret_cast<const String*>(reinterpret_cast<const char *>(this) + offset);
+        if (str->str.size == 0)
+            return QString();
         QStringDataPtr holder = { const_cast<QStringData *>(static_cast<const QStringData*>(&str->str)) };
         QString qstr(holder);
         if (flags & StaticData)
diff --git a/src/qml/qml/qqmlproperty.cpp b/src/qml/qml/qqmlproperty.cpp
index 0bbcafda54..1075b53c5e 100644
--- a/src/qml/qml/qqmlproperty.cpp
+++ b/src/qml/qml/qqmlproperty.cpp
@@ -1403,6 +1403,12 @@ bool QQmlPropertyPrivate::write(QObject *object,
             v = value;
             if (v.convert(propertyType)) {
                 ok = true;
+            } else if (v.isValid() && value.isNull()) {
+                // For historical reasons converting a null QVariant to another type will do the trick
+                // but return false anyway. This is caught with the above condition and considered a
+                // successful conversion.
+                Q_ASSERT(v.userType() == propertyType);
+                ok = true;
             } else if ((uint)propertyType >= QVariant::UserType && variantType == QVariant::String) {
                 QQmlMetaType::StringConverter con = QQmlMetaType::customStringConverter(propertyType);
                 if (con) {